Chapter 6, System Design Lecture 1

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Chapter 6, System DesignLecture 1
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Design

• There are two ways of constructing a software
  design One way is to make it so simple that
  there are obviously no deficiencies, and the
  other way is to make it so complicated that there
  are no obvious deficiencies.
• - C.A.R. Hoare

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Why is Design so Difficult?

• Analysis Focuses on the application domain
• Design Focuses on the implementation domain
• Design knowledge is a moving target
• The reasons for design decisions are changing
  very rapidly
• Halftime knowledge in software engineering About
  3-5 years
• What I teach today will be out of date in 3 years
• Cost of hardware rapidly sinking
• Design window
• Time in which design decisions have to be made

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The Purpose of System Design
Problem

• Bridging the gap between desired and existing
  system in a manageable way
• Use Divide and Conquer
• We model the new system to be developed as a set
  of subsystems

New System
Existing System
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System Design
System Design
Failure
2. System
Decomposition
Layers/Partitions Coherence/Coupling
7. Software Control
Monolithic Event-Driven Threads Conc. Processes
3. Concurrency
6. Global

4. Hardware/
Identification of Threads
5. Data
Resource Handling
Softwar
e

Management
Mapping
Access control Security
Persistent Objects
Special purpose
Files
Buy or Build Trade-off
Databases
Allocation
Data structure
Connectivity
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Overview

• System Design I
• 0. Overview of System Design
• 1. Design Goals
• 2. Subsystem Decomposition
• System Design II (next lecture)
• 3. Concurrency
• 4. Hardware/Software Mapping
• 5. Persistent Data Management
• 6. Global Resource Handling and Access Control
• 7. Software Control
• 8. Boundary Conditions

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How to use the results from the Requirements
Analysis for System Design

• Nonfunctional requirements gt
• Activity 1 Design Goals Definition
• Use Case model gt
• Activity 2 System decomposition (Selection of
  subsystems based on functional requirements,
  coherence, and coupling)
• Object model gt
• Activity 4 Hardware/software mapping
• Activity 5 Persistent data management
• Dynamic model gt
• Activity 3 Concurrency
• Activity 6 Global resource handling
• Activity 7 Software control
• Activity 8 Boundary conditions

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Section 1. Design Goals

• Reliability
• Modifiability
• Maintainability
• Understandability
• Adaptability
• Reusability
• Efficiency
• Portability
• Traceability of requirements
• Fault tolerance
• Backward-compatibility
• Cost-effectiveness
• Robustness
• High-performance

• Good documentation
• Well-defined interfaces
• User-friendliness
• Reuse of components
• Rapid development
• Minimum of errors
• Readability
• Ease of learning
• Ease of remembering
• Ease of use
• Increased productivity
• Low-cost
• Flexibility

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Relationship Between Design Goals
End User
Functionality User-friendliness Ease of Use Ease
of learning Fault tolerant Robustness
Low cost Increased Productivity Backward-Compatib
ility Traceability of requirements Rapid
development Flexibility
Runtime Efficiency
Reliability
Portability Good Documentation
Client
(Customer,
Sponsor)
Minimum of errors Modifiability,
Readability Reusability, Adaptability Well-defined
interfaces
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Typical Design Trade-offs

• Functionality vs. Usability
• Cost vs. Robustness
• Efficiency vs. Portability
• Rapid development vs. Functionality
• Cost vs. Reusability
• Backward Compatibility vs. Readability

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Nonfunctional Requirements give a clue for the
use of Design Patterns

• Read the problem statement again
• Use textual clues (similar to Abbots technique
  in Analysis) to identify design patterns
• Text manufacturer independent, device
  independent, must support a family of products
• Abstract Factory Pattern
• Text must interface with an existing object
• Adapter Pattern
• Text must deal with the interface to several
  systems, some of them to be developed in the
  future, an early prototype must be
  demonstrated
• Bridge Pattern

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Textual Clues in Nonfunctional Requirements

• Text complex structure, must have variable
  depth and width
• Composite Pattern
• Text must interface to an set of existing
  objects
• Façade Pattern
• Text must be location transparent
• Proxy Pattern
• Text must be extensible, must be scalable
• Observer Pattern
• Text must provide a policy independent from the
  mechanism
• Strategy Pattern

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Section 2. System Decomposition

• Subsystem (UML Package)
• Collection of classes, associations, operations,
  events and constraints that are interrelated
• Seed for subsystems UML Objects and Classes.
• Service
• Group of operations provided by the subsystem
• Seed for services Subsystem use cases
• Service is specified by Subsystem interface
• Specifies interaction and information flow
  from/to subsystem boundaries, but not inside the
  subsystem.
• Should be well-defined and small.
• Often called API Application programmers
  interface, but this term should used during
  implementation, not during System Design

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Services and Subsystem Interfaces

• Service A set of related operations that share a
  common purpose
• Notification subsystem service
• LookupChannel()
• SubscribeToChannel()
• SendNotice()
• UnscubscribeFromChannel()
• Services are defined in System Design
• Subsystem Interface Set of fully typed related
  operations. Also called application programmer
  interface (API)
• Subsystem Interfaces are defined in Object Design

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Choosing Subsystems

• Criteria for subsystem selection Most of the
  interaction should be within subsystems, rather
  than across subsystem boundaries (High
  coherence).
• Does one subsystem always call the other for the
  service?
• Which of the subsystems call each other for
  service?
• Primary Question
• What kind of service is provided by the
  subsystems (subsystem interface)?
• Secondary Question
• Can the subsystems be hierarchically ordered
  (layers)?
• What kind of model is good for describing layers
  and partitions?

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Example STARS Subsystem Decomposition
Is this the right decomposition or is this too
much ravioli?
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Definition Subsystem Interface Object

• A Subsystem Interface Object provides a service
  
• This is the set of public methods provided by the
  subsystem
• The Subsystem interface describes all the methods
  of the subsystem interface object
• Use a Facade pattern for the subsystem interface
  object

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STARS as a set of subsystems communicating via a
software bus
Authoring
Modeling
Workflow
Augmented Reality
Inspection
Repair
Workorder
A Subsystem Interface Object publishes the
service ( Set of public methods) provided by
the subsystem
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STARS as a 3-layered Architecture
What is the relationship between Modeling and
Authoring? Are other subsystems needed?
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Coupling and Coherence

• Goal Reduction of complexity
• Coherence measures the dependence among classes
• High coherence The classes in the subsystem
  perform similar tasks and are related to each
  other (via associations)
• Low coherence Lots of misc and aux objects, no
  associations
• Coupling measures dependencies between subsystems
• High coupling Modifications to one subsystem
  will have high impact on the other subsystem
  (change of model, massive recompilation, etc.)
• Subsystems should have as maximum coherence and
  minimum coupling as possible
• How can we achieve loose coupling?
• Which subsystems are highly coupled?

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Partitions and Layers

• A large system is usually decomposed into
  subsystems using both, layers and partitions.
• Partitions vertically divide a system into
  several independent (or weakly-coupled)
  subsystems that provide services on the same
  level of abstraction.
• A layer is a subsystem that provides services to
  a higher level of abstraction
• A layer can only depend on lower layers
• A layer has no knowledge of higher layers

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Subsystem Decomposition into Layers

• Subsystem Decomposition Heuristics
• No more than 7/-2 subsystems
• More subsystems increase coherence but also
  complexity (more services)
• No more than 5/-2 layers

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Layer and Partition Relationships between
Subsystems

• Layer relationship
• Layer A Calls Layer B (runtime)
• Layer A Depends on Layer B (make dependency,
  compile time)
• Partition relationship
• The subsystem have mutual but not deep knowledge
  about each other
• Partition A Calls partition B and partition B
  Calls partition A

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Virtual Machine (Dijkstra, 1965)

• A system should be developed by an ordered set of
  virtual machines, each built in terms of the ones
  below it.

Problem
VM1
C1
C1
C1
attr
attr
attr
opr
opr
opr
C1
C1
VM2
attr
attr
opr
opr
C1
VM3
C1
attr
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opr
opr
C1
VM4
attr
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Existing System
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Virtual Machine

• A virtual machine is an abstraction that provides
  a set of attributes and operations.
• A virtual machine is a subsystem connected to
  higher and lower level virtual machines by
  "provides services for" associations.
• Virtual machines can implement two types of
  software architecture closed and open
  architectures.

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Closed Architecture (Opaque Layering)

• A virtual machine can only call operations from
  the layer below
• Design goal High maintainability

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Open Architecture (Transparent Layering)

• A virtual machine can call operations from any
  layers below
• Design goal Runtime efficiency

VM1
VM2
VM3
VM4
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Properties of Layered Systems

• Layered systems are hierarchical. They are
  desirable because hierarchy reduces complexity.
• Closed architectures are more portable.
• Open architectures are more efficient.
• If a subsystem is a layer, it is often called a
  virtual machine.
• Layered systems often have a chicken-and egg
  problem
• Example Debugger opening the symbol table when
  the file system needs to be debugged

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Software Architectures

• Subsystem decomposition
• Identification of subsystems, services, and their
  relationship to each other.
• Specification of the system decomposition is
  critical.
• Patterns for software architecture
• Client/Server Architecture
• Peer-To-Peer Architecture
• Repository Architecture
• Model/View/Controller
• Pipes and Filters Architecture

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Client/Server Architecture

• One or many servers provides services to
  instances of subsystems, called clients.
• Client calls on the server, which performs some
  service and returns the result
• Client knows the interface of the server (its
  service)
• Server does not need to know the interface of the
  client
• Response in general immediately
• Users interact only with the client

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Client/Server Architecture

• Often used in database systems
• Front-end User application (client)
• Back end Database access and manipulation
  (server)
• Functions performed by client
• Customized user interface
• Front-end processing of data
• Initiation of server remote procedure calls
• Access to database server across the network
• Functions performed by the database server
• Centralized data management
• Data integrity and database consistency
• Database security
• Concurrent operations (multiple user access)
• Centralized processing (for example archiving)

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Design Goals for Client/Server Systems

• Portability
• Server can be installed on a variety of machines
  and operating systems and functions in a variety
  of networking environments
• Transparency
• The server might itself be distributed (why?),
  but should provide a single "logical" service to
  the user
• Performance
• Client should be customized for interactive
  display-intensive tasks
• Server should provide CPU-intensive operations
• Scalability
• Server has spare capacity to handle larger number
  of clients
• Flexibility
• Should be usable for a variety of user interfaces
• Reliability
• System should survive individual node and/or
  communication link problems

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Problems with Client/Server Architectures

• Layered systems do not provide peer-to-peer
  communication
• Peer-to-peer communication is often needed
• Example Database receives queries from
  application but also sends notifications to
  application when data have changed

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Peer-to-Peer Architecture

• Generalization of Client/Server Architecture
• Clients can be servers and servers can be clients
• More difficult because of possibility of deadlocks

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Example of a Peer-to-Peer Architecture

• ISOs OSI Reference Model
• ISO International Standard Organization
• OSI Open System Interconnection
• Reference model defines 7 layers of network
  protocols and strict methods of communication
  between the layers.

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Middleware Allows You To Focus On The Application
Layer
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Model/View/Controller

• Subsystems are classified into 3 different types
• Model subsystem Responsible for application
  domain knowledge
• View subsystem Responsible for displaying
  application domain objects to the user
• Controller subsystem Responsible for sequence
  of interactions with the user and notifying views
  of changes in the model.
• MVC is a special case of a repository
  architecture
• Model subsystem implements the central
  datastructure, the Controller subsystem
  explicitly dictate the control flow

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Example of a File System based on MVC
Architecture
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Sequence of Events
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Repository Architecture

• Subsystems access and modify data from a single
  data structure
• Subsystems are loosely coupled (interact only
  through the repository)
• Control flow is dictated by central repository
  (triggers) or by the subsystems (locks,
  synchronization primitives)

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Examples of Repository Architecture
Compiler
SyntacticAnalyzer
Optimizer
CodeGenerator
LexicalAnalyzer

• Hearsay II speech understanding system
  (Blackboard architecture)
• Database Management Systems
• Modern Compilers

SyntacticEditor
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Summary

• System Design
• Reduces the gap between requirements and the
  machine
• Decomposes the overall system into manageable
  parts
• Design Goals Definition
• Describes and prioritizes the qualities that are
  important for the system
• Defines the value system against which options
  are evaluated
• Subsystem Decomposition
• Results into a set of loosely dependent parts
  which make up the system